The present invention relates to offshore seismic exploration and particularly to an apparatus for decoupling the seismic cable from the tow vessel. In an offshore seismic operation, a seismic cable having a length of one to three miles and containing a large number of hydrophones is towed behind a vessel to detect seismic signals. The detected seismic signals are the result of seismic waves produced by a seismic source being reflected or refracted from underground formations. Since the reflected signals are low level signals, the hydrophones must be sensitive and thus are also sensitive to any extraneous noise. The extraneous noise is in part due to the sea background noise, sea traffic, water flow past the cable and noise transmitted from the tow vessel. The present invention is directed primarily to eliminating the latter souce of noise.
As the tow vessel moves through the sea, it pitches, heaves and rolls due to the ocean swells and waves and this motion is transmitted to the tow cable that is connected to the seismic cable. At times the transmitted motion can have high acceleration as, for instance, when the tow cable slams against the tow vessel. In addition to the motions induced in the tow cable by the two vessel's motion, additional motion is produced by the vortex shedding that occurs as the cable is moved through the water. Vortex shedding causes the tow cable to oscillate or vibrate and occurs whenever the angle between the two cable and the water flow exceeds a critical angle.